Process for improving surface finish on clad aluminum sheets

ABSTRACT

A buffing process for improving the surface finish of clad aluminum-alloy sheets. Aluminum sheet stock is burnished according to a specific process program to provide uniform surface appearance and improved corrosion resistance, and the process provides controlled removal of a thin surface layer of cladding used in aircraft-skin aluminum sheets.

United States Patent Queyrel et al.

PROCESS FOR IMPROVING SURFACE FINISH ON CLAD ALUMINUM SHEETS Paul J.Queyrel, Newport Beach; Jerome O. Koenig, Culver City, both of Calif.

Mill Polishing Corporation, Huntington Park, Calif.

Filed: Nov. 25, 1970 Appl. No.: 92,562

lnventors:

Field ofSearch ..51/3l73l8, 328, 51/283, 45,142, 51, 38, 358

[ 51 Mar. 28, 1972 [56] References Cited UNITED STATES PATENTS 2,293,5158/1942 Littwin ..5 H45 2,436,466 2/1948 Wilson.... ....51/45 3,553,901l/1971 Burt ..51/142 Primary Examiner-Donald G. Kelly Attorney-Christie,Parker & Hale [57] ABSTRACT A buffing process for improving the surfacefinish of clad aluminum-alloy sheets. Aluminum sheet stock is burnishedaccording to a specific process program to provide uniform surfaceappearance and improved corrosion resistance, and the process providescontrolled removal of a thin surface layer of cladding used inaircraft-skin aluminum sheets.

6 Claims, 4 Drawing Figures Patented March 28, 1972 3,651,604

m ii iiiiiii lllll 1w '1" filiililllll mnziq 39 1 num W INVENTORS.

PROCESS FOR IMPROVING SURFACE FINISH ON CLAD ALUMINUM SHEETS BACKGROUNDOF THE INVENTION Clad aluminum sheets such as alloy 2024T3 are in wideuse in aircraft construction, and are formed as an aluminum-alloy coreor base sheet to which is metallurgically bonded a protective coating orcladding of high-purity aluminum or an alloy of aluminum. The base-sheetalloy is selected to provide the desired mechanical properties in theoverall sheet, and the cladding alloy is selected to provide corrosionprotection for the underlying base sheet.

Ordinary mill run clad-aluminum sheets are generally not suitable forairframe construction as these sheets lack a uniform finely finishedsurface which is desired for aircraft exterior surfaces. Airframemanufacturers have instead tended to select premium-finish sheets whichreceive additional mill treatment with finish cold-rolling equipment andother processes. This premium-finish treatment adds appreciably to theprice of the aluminum stock as the production yield of satisfactorysheets is relatively low, and the additional processing is a significantincrease in factory cost.

Buffing processes have been used in the past on aircraft aluminum stock,but these skin materials have typically been heavy sheets or plates(sometimes chemically milled for weight reduction) with thick claddingwhich could withstand heavy abrasive treatment without exposing the basealloy. Thin clad-aluminum sheet is demanded for construction of modernaircraft, and this stock may have a cladding thickness of only a fewthousandths of an inch. Conventional buffing techniques have provedunsatisfactory with these thin sheets due to excessive removal ofcladding which may expose the base material and in any event providesinadequate corrosion resistance.

We have developed a buffing or burnishing process which achievesaircraft-quality finishes on mill run sheets with a reduction incladding thickness of only about 0.0003 inch. Sheets processed accordingto the invention exhibit a superior surface finish of uniform color, andminor scuffs, scratches, and blemishes such as oil stains are removedfrom the mill run stock. The processed sheets further exhibit improvedcorrosion resistance which is believed due to a micro-levelling andcompaction of the cladding which results from the buffing process. Yieldof satisfactory sheets from the process is high and uniform, andaircraft specifications are met at a cost substantially lower than thatof sheets finished by premium coldrolling treatment.

SUMMARY OF THE INVENTION The process of this invention first involvespositioning a clad-aluminum sheet in a buffing machine, the machinehaving a rotatable motor-driven cloth buff of an axial length adequateto span the sheet. The buff is rotated to maintain a buff surface speedin the range of 3,750 to 4,450 surface feet per minute. The buff ismoved against the sheet while maintaining relative motion of the sheetand buff in a direction normal to the buff rotation axis to pass thebuff across the sheet at a substantially constant lineal feed rate. Aslight oscillatory relative motion of the buff and sheet is alsomaintained during buffing, and this oscillatory motion is about onecycle per second in a direction generally parallel to the buff rotationaxis. A liquid buffing compound is applied to the buff and sheetthroughout the buffing process, and dwell time of the buff on anyportion of the sheet is limited to about 6 to 7 seconds.

Preferably, the lineal feet rate of the sheet is maintained in a rangeof 25 to 35 feet per minute, and the buff is urged against the sheetwith a force which produces a drive-motor load of about one-quarterhorsepower per inch of buff-sheet contact in a direction parallel to thebuff rotation axis. This force fiattens the buff to produce a contactsurface of about 3-inches width between buff and sheet across the fullwidth of the sheet. The buff preferably has a diameter in the range ofabout 16 to 24 inches, and is formed of alternating disks of cloth andpaper mounted between fiber spacers on a spindle, the paper disks havinga diameter less than the minimum diameter of the buff to remain out ofcontact with the sheet.

DESCRIPTION OF THE DRAWINGS FIG. 1 is a side view of a buffing machinesuitable for use according to the method of this invention;

FIG. 2 is an end view of the machine taken from the left side of FIG. 1;

FIG. 3 is a side view of a buff-wheel disk; and 1 FIG. 4 is a sectionalelevation of a portion of a buff-wheel assembly.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to the drawings, aburnishing or buffing machine 10 includes a table assembly 11 supportedby a plurality of wheels 12 on a pair of rails 13 secured to a levelfioor 14. A steel plate 15 forms the top surface of the table assembly,and the plate is stiffened by lateral cross members 16 and longitudinalmembers (not shown) to insure a level top surface. The table assemblymust be quite large to accommodate clad-aluminum sheets as used inaircraft construction, and plate 15 is typically about 44 feet long and10 feet wide.

As is conventional in buffing machines for sheet stock, a drive means(not shown) is provided to drive the table assembly along the supportingrails. A satisfactory drive means is a rotary hydraulic motor secured tothe floor between the rails and having an output cog engaged with adrive chain or rack secured to the table. Electric motors or other drivesystems of conventional types can also be used to drive the table at adesired speed of about 25 to 35 feet per minute during sheet buffing.

A conventional friction clamp 18 extends across the width of one end ofplate 15, and a clad-aluminum sheet 19 to be buffed is secured at oneedge in the clamp. A slightly resilient pad 20 ispositioned between theundersurface of sheet 19 and the top of plate 15 to protect the bottomof the sheet and to provide a slight cushion during buffing. Preferably,pad 20 is an inexpensive expendable material such as a compressedwood-fiber sheet known in the construction industry as Upson Board.

A buff-rake assembly 23 is secured to an end of plate 15 remote fromclamp 18, and is used to clean and condition the buff (described below)prior to finishing of sheet 19. Assembly 23 includes a coarse rake 24formed as a plurality of upstanding pins extending across the full widthof plate 15, a serrated curry comb 25 used for intermediate dressing,and a fine rake 26 (preferably 24-grit open-coat aluminum oxideabrasive) for final dressing of the buff.

A buffing-head base 30 includes a pair of generally U- shaped portions31 on opposite sides of table assembly 11 and each having upstandingmembers 32 and 33. Portions 31 are secured to floor l4, and the upperends of members 32 are tied together by a cross beam 34. A buffing-headframe 35 is pivotally mounted to base 30 by a hinge 36 extending alongthe length of cross beam 34. Hinge 36 has several inches of axialfreedom so the buffing-head frame can be oscillated sideways duringbuffing as described below.

A hydraulic cylinder 39 is secured to each member 33 of the base, and apiston rod 40 extending from each cylinder is secured to buffing-headframe 35 by a limited-motion fitting (not shown) which permits thesideways oscillatory motion of the frame mentioned above. The frame israised or lowered by actuating cylinders 39 to pivot the frame aroundhinge 36. A pair of adjustable stops 41 are secured to opposite sides offrame 35, and a ball 42 at the lower end of each stop rests and rollsfreely on a level upper surface of pedestallike member 33 to permitoscillatory motion of the frame. A screw jack (not shown) or similaradjustable means is provided within stop 41 to vary the maximum downwardtravel of frame 35 toward the upper surface of members 33. A motor 43 issecured to frame 35 adjacent hinge 36, and an output cam (not shown) onthe motor bears on one of members 32 to oscillate the frame sideways orlaterally with respect to the direction of motion of table assembly 11.This oscillatory motion is preferably about one cycle per second at apeak-to-peak amplitude of about 1 inch.

A buff-wheel assembly 45 is mounted on a spindle 46 by a pair of bearingsupports 47 secured at opposite sides of frame 35. A drive motor 48 ismounted on a pad 49 on frame 35, and is coupled to the buff-wheelassembly by a plurality of drive belts 50.

A rail 52 is secured across the front of frame 35, and a spray-headassembly 53 is movably mounted on the rail to move across the surface ofbuff-wheel assembly 45. A liquid buffing compound is delivered to thespray head by a line 54, and the head is reciprocated across rail 52during a buffing operation by a conventional drive mechanism (not shown)to spray the liquid compound across the buff.

Buff-wheel assembly 45 is shown in detail in FIGS. 3 and 4. The assemblycomprises a plurality of buffing disks 60 having a central opening 61with opposed keyways 62. Spindle 46 fits through opening 61 in eachdisk, and keys (not shown) which extend the length of the spindle engagekeyways 62. Adjacent buffing disks 60 as mounted on spindle 46 areseparated by hard fiber rings or spacers 63 of about one-sixteenth inchthickness. The spacers allow the buff disks to spread slightly to imparta desirable softness to the buff surface.

Each buffing disk 60 is preferably made of 16 plys of 86/93 cottonbuff-cloth disks 65 (FIG. 3) having a weight of about 2.5 ounces peryard. The individual cloth plys are separated by a 40-pound kraft-paperdisk 66, and the cloth and paper disks are secured together by threerows of stitching 67. The cloth disks have a typical outside diameter of24 inches, and a central opening of about -inch diameter. Thekraft-paper disks and fiber spacers have an outside diameter of aboutinches to remain well below the buff surface as the overall diameter ofthe buff-wheel assembly decreases due to wear.

Bufflng disks 60 have a thickness of about one-fourth inch, and 300 to400 disks are needed to make a buff-wheel assembly of say 10 feet inlength to span the large aluminum sheets used in aircraft construction.The buffing disks and fiber spacers are stacked individually on spindle46, and are compressed tightly together and locked in place by clamps(not shown) at the ends ofthe spindle.

In use of the process, buff-wheel assembly 45 is first raked, smoothedand dressed by passing buff-rake assembly 23 beneath the buffing wheel.The table-drive system is then actuated to feed sheet 19 beneath thebuffing-wheel assembly, We have found that the lineal feed rate of thesheet should be maintained substantially constant in the range of 25 to35 feet per minute. Excessive sheet heating may occur with lineal feedrates less than 25 feet per minute, and an inferior sheet surface mayresult from feed rates in excess of 35 feet per minute due to stringlines arising from loose threads in the buff.

As the buffing operation is commenced, a water-based liquid buffingcompound is delivered to spray head 53 for application to the buff-wheelassembly. Preferably, this compound is an unfused aluminum oxide of 320grit or finer in a tallow vehicle. A satisfactory compound is availableunder the designation M-ll buffing compound from M & T Chemical Company,Rahway, New Jersey. The amount of compound applied is controlled by themachine operator, and compound flow may be increased to provide extrabuffing action to remove oil stains or other surface irregularities.

The speed of drive motor 48 is controlled to maintain the surface speedof the buff-wheel assembly in the range of3,750 to 4,450 surface feetper minute. Surface speeds less than the lower end of this range resultin inferior buffing because the edges of the individual buffing diskstend to bend and deflect, and to lose their burnishing action. Surfacespeeds above the upper end of the range tend to harden the buff,preventing following of slight surface irregularities in the sheet andcausing excessive removal of clad material from the surface beingbufi'ed. The speed of drive motor 48 is adjusted as the buff wears tomaintain the surface speed in the desired range. A substantial variationin motor speed may be needed, as a buff having an original diameter of24 inches may be used down to a diameter of about 16 inches before itmust be discarded.

As the process is commenced, hydraulic cylinders 39 are actuated toretract piston rods 40 and thereby lower hinged frame 35 to urge thebuff-wheel assembly against sheet 19. Stops 41 are adjusted in heightuntil the surface of the buffwheel assembly which contacts the aluminumsheet is flat tened or formed as a chord of the otherwise circular buffover a distance of about 3 inches. That is, the buff is urged againstthe sheet with a force sufficient to cause simultaneous contact of thebuff-wheel assembly with a 3-inch length of the sheet across the fullspan of the sheet. This flattened surface of the buff should bemaintained in a range of about two and onehalf to three and one-halfinches. The load on drive motor 48 increases as the extent of buffflattening is increased, and monitoring of motor load using conventionalgages is a convenient way of adjusting this parameter. We have foundthat motor loads of 0.22 to 028 horsepower per inch of sheet widthproduce the proper degree of buff pressure and hence buff flattening.

The degree of buff flattening and the lineal feed rate of the aluminumsheet set the dwell time of the buff on any given portion of the sheetduring each pass of table assembly 11. [t is important that this contactor dwell time be limited to a total of about 6 to 7 seconds during thebuffing operation, and this maximum limit is typically achieved in about5 back-and-forth passes of the sheet beneath the buff-wheel assembly.Total dwell times beyond this limit may produce an excessive removal ofclad material which cannot be tolerated in thinly clad aluminum sheetsused in modern aircraft construction.

Lateral oscillator motor 43 is operated throughout the buffing processto provide a relative lateral oscillatory motion of the sheet andbuff-wheel assembly. This motion has a double amplitude of about 1 inchpeak-to-peak, and a frequency of about one cycle per second.Substantially lower oscillation frequency should be avoided to preventlateral deflection of the buff disks, and substantially higherfrequencies may produce excessive vibration.

The sheet is passed beneath the buff wheel assembly a sufficient numberof times to achieve the desired finish, while still observing themaximum dwell time of about 6 to 7 seconds. We have found that removalof cladding can be held to about 0.0003 inch if the above-describedprocess parameters are carefully observed. The finished sheet hassuperior surface characteristics with uniform color, and oil stains andother surface blemishes are removed. These factors are all of importancein aircraft construction where adjoining sheets in the air frame shouldhave the same surface appearance to avoid a checkerboard pattern. 1

There has been described a process for burnishing or buffing mill runclad-aluminum sheets. The process produces a premium finish on thesheets, and achieves this objective with minimum removal of protectivecladding from the sheet. We have also found that sheets buffed accordingto the process of this invention exhibit an improved resistance tocorrosion arising from exposure to sea-coast air or other cor rosiveatmospheres. It is believed that the buffing process tightens orcompacts the clad surface, and reduces the number of pockets or tinypores in which corrosion typically begins.

What is claimed is:

1. A process for burnishing a clad surface of a clad-aluminum sheet toachieve uniform surface appearance and improved corrosion resistancewith minimum clad removal, comprising the steps of:

a. positioning the sheet in a buffing machine having a rotatablemotor-driven cloth buff of an axial length adequate to span the sheetand overlap the sheet edges;

b. rotating the buff to maintain a buff surface speed of about 4,000surface feet per minute during the performance of the remaining processsteps;

c. moving the buff against the sheet while maintaining relative motionof the sheet and buff in a direction normal to the buff rotation axis topass the buff across the sheet at a substantially constant lineal feedrate, and while maintaining a slight oscillatory relative motion of thebuff and sheet in a direction generally parallel to the buff rotationaxis; I

d. maintaining a liquid buffing compound at an interface of the buff andsheet throughout the buffing;

e. limiting dwell time of the buff on any portion of the sheet to about6 to 7 seconds.

2. The process defined in claim 1 in which the buff surface speed isheld within a range of 3,750 to 4,450 surface feet per minute.

3. The process defined in claim 2 in which the lineal feed rate is heldwithin a range of 25 to 35 feet per minute, and separate buffing passesare repeated to achieve said dwell time.

4. The process defined in claim 3 in which the buff is urged against thesheet with a force which produces a buff-driving motor load of aboutone-quarter horsepower per inch of buffsheet contact in a directionparallel to the buff rotation axis.

5. The process defined in claim 3 in which the buff has a diameter inthe range of about 16 to 24 inches and is formed of alternating disks ofcloth and paper mounted on a spindle, the paper disks having a diameterless than the minimum diameter of the buff to remain out of contact withthe sheet, and in which the buff is urged against the sheet withsufiicient force to produce simultaneous contact of the buff and sheetover a distance of about 3 inches in the direction of lineal feed.

6. The process defined in claim 5 in which the relative oscillatorymotion of the buff and sheet is about one cycle per second at about oneinch peak-to-peak displacement.

1. A process for burnishing a clad surface of a clad-aluminum sheet toachieve uniform surface appearance and improved corrosion resistancewith minimum clad removal, comprising the steps of: a. positioning thesheet in a buffing machine having a rotatable motor-driven cloth buff ofan axial length adequate to span the sheet and overlap the sheet edges;b. rotating the buff to maintain a buff surface speed of about 4,000surface feet per minute during the performance of the remaining processsteps; c. moving the buff against the sheet while maintaining relativemotion of the sheet and buff in a direction normal to the buff rotationaxis to pass the buff across the sheet at a substantially constantlineal feed rate, and while maintaining a slight oscillatory relativemotion of the buff and sheet in a direction generally parallel to thebuff rotation axis; d. maintaining a liquid buffing compound at aninterface of the buff and sheet throughout the buffing; e. limitingdwell time of the buff on any portion of the sheet to about 6 to 7seconds.
 2. The process defined in claim 1 in which the buff surfacespeed is held within a range of 3,750 to 4,450 surface feet per minute.3. The process defined in claim 2 in which the lineal feed rate is heldwithin a range of 25 to 35 feet per minute, and separate buffing passesare repeated to achieve said dwell time.
 4. The process defined in claim3 in which the buff is urged against the sheet with a force whichproduces a buff-driving motor load of about one-quarter horsepower perinch of buff-sheet contact in a direction parallel to the buff rotationaxis.
 5. The process defined in claim 3 in which the buff has a diameterin the range of about 16 to 24 inches and is formed of alternating disksof cloth and paper mounted on a spindle, the paper disks having adiameter less than the minimum diameter of the buff to remain out ofcontact with the sheet, and in which the buff is urged against the sheetwith sufficient force to produce simultaneous contact of the buff andsheet over a distance of about 3 inches in the direction of lineal feed.6. The process defined in claim 5 in which the relative oscillatorymotion of the buff and sheet is about one cycle per second at about oneinch peak-to-peak displacement.